The invention generally relates to pacemakers or other implantable cardiac stimulation devices and in particular to techniques for reducing the effects of evoked response bleed-through on electrical polarization measurements performed in connection with calibrating an automatic capture system of an implantable cardiac stimulation device, e.g., a pacemaker, an implantable cardioverter/defibrillator (ICD), or the like.
DESCRIPTION OF RELATED ART
A pacemaker is a medical device, typically implanted within a patient, that provides electrical stimulation pulses to selected chambers of the heart, i.e., the atria and/or the ventricles. Such stimulation pulses cause the muscle tissue of the heart (myocardial tissue) to depolarize and contract, thereby causing the heart to beat at a controlled rate.
Most pacemakers can be programmed to operate in a demand mode of operation, i.e., to generate and deliver stimulation pulses to the heart only when the heart fails to beat on its own. To this end, the pacemaker senses cardiac activity, i.e., heart beats, and if the heart beats do not occur at a prescribed rate, then stimulation pulses are generated and delivered to an appropriate heart chamber, either an atrium or a ventricle, in order to force the heart to beat.
When operating in a demand mode of operation, the pacemaker defines a period of time, referred to generally as the xe2x80x9cescape intervalxe2x80x9d (which may further be referred to as either an xe2x80x9catrial escape intervalxe2x80x9d or a xe2x80x9cventricular escape interval,xe2x80x9d depending upon the mode of operation of the pacemaker) that is slightly longer than the period of time between normal heart beats. Upon sensing such a xe2x80x9cnaturalxe2x80x9d (non-stimulated or non-paced) heart beat within the allotted time period, the escape interval is reset, and a new escape interval is started. A stimulation (or pacing) pulse is generated at the conclusion of this new escape interval unless a natural heart beat is again sensed during the escape interval. In this way, stimulation pulses are generated xe2x80x9con demand,xe2x80x9d i.e., only when needed to maintain the heart rate at a rate that never drops below the rate set by the escape interval.
The heart rate is monitored by examining the electrical signals that are manifest concurrent with the depolarization or contraction of the myocardial tissue. The contraction of atrial muscle tissue is manifest by the generation of a P-wave. The contraction of ventricular muscle tissue is manifest by the generation of an R-wave (sometimes referred to as the xe2x80x9cQRS complexxe2x80x9d). The sequence of electrical signals that represent P-waves followed by R-waves (or QRS complexes) can be sensed from inside of or proximate to the heart by using sensing leads implanted inside or on the heart, e.g., pacemaker leads, or by using external electrodes attached to the skin of the patient.
Most modern implantable pacemakers are programmable. That is, the basic escape interval (atrial and/or ventricular) of the pacemaker, as well as the sensitivity (threshold level) of the sensing circuits used in the pacemaker to sense P-waves and/or R-waves, and numerous other operating parameters of the pacemaker, may be programmably set at the time of implantation or thereafter to best suit the needs of a particular patient. Hence, the pacemaker can be programmed so as to yield a desired performance.
The operation of a pacemaker as described above presupposes that a stimulation pulse generated by the pacemaker effectuates capture. As used herein, the term xe2x80x9ccapturexe2x80x9d refers to the ability of a given stimulation pulse generated by a pacemaker to cause depolarization of the myocardium, i.e., to cause the heart muscle to contract, or to cause the heart to xe2x80x9cbeat.xe2x80x9d A stimulation pulse that does not capture the heart is thus a stimulation pulse that may just as well have not been generated, since it has not caused the heart to beat. Such a non-captured stimulation pulse not only represents wasted energyxe2x80x94energy drawn from a limited energy resource (e.g., a battery) of the pacemakerxe2x80x94but worse still may provide the pacemaker logic circuits with false information. That is, the logic circuits of the pacemaker may presuppose that each stimulation pulse generated by the pacemaker captured the heart. If the stimulation pulse does not capture the heart, then the pacemaker logic circuits control the operation of the pacemaker may be based on false information, and may thus control the pacemaker in an inappropriate manner. Thus, there is a critical need for a pacemaker to properly determine whether a given stimulation pulse has effectuated capture.
While there are many factors that influence whether a given stimulation pulse effectuates capture, a principal factor is the energy of the stimulation pulse. The energy of the stimulation pulse, in turn, is determined by the amplitude and width of the stimulation pulse generated by the pacemaker. Advantageously, in a programmable pacemaker, both the amplitude and pulse width of the stimulation pulse are parameters that may be programmably controlled or set to a desired value.
An implantable pacemaker derives its operating power, including the power to generate a stimulation pulse, from a battery. The power required to repeatedly generate a stimulation pulse dominates the total power consumed by a pacemaker. Hence, to the degree that the power associated with the stimulation pulse can be minimized, the life of the battery can be extended and/or the size and weight of the battery can be reduced. Unfortunately, however, if the power associated with a stimulation pulse is reduced too far, the stimulation pulse is not able to consistently effectuate capture, and the pacemaker is thus rendered ineffective at performing its intended function. Thus, it is desirable for a pacemaker to adjust the energy of a stimulation pulse to an appropriate level that provides sufficient energy to effectuate capture, i.e., generate an evoked response, but does not expend any significant energy beyond that required to effectuate capture.
Initially, the most common technique used to adjust the stimulation energy to an appropriate level was manual, using the programmable features of the pacemaker. That is, at the time of implant, a cardiologist or other physician conducts some preliminary stimulation tests to determine how much energy a given stimulation pulse must have to effectuate capture at a given tissue location. If the preliminary tests indicate that the capture threshold is high (compared to the average patient) then the lead will be repositioned until a xe2x80x9cgoodxe2x80x9d threshold is found. Once it has been determined that the thresholds are acceptable, the stimulation electrode is then left in place and the amplitude and/or width of the stimulation pulse is set to a level that is typically 2 to 3 times greater than the amplitude and/or width determined in the preliminary tests. The increase in energy above and beyond the energy needed to effectuate capture is considered as a xe2x80x9csafety margin.xe2x80x9d
During the acute phase, e.g., over a period of days or weeks after implant, the stimulation pulse energy needed to effectuate capture usually changes. This stimulation pulse energy is hereafter referred to as the xe2x80x9ccapture-determining threshold.xe2x80x9d Hence, having a safety margin factored into the stimulation pulse energy allows the stimulation pulses generated by the pacemaker to continue to effectuate capture despite changes in the capture-determining threshold. Unfortunately, however, much of the energy associated with the safety margin represents wasted energy, and shortens the life of the battery. Furthermore, after the acute phase (when the lead is considered in the chronic phase), the capture-determining threshold is typically much lower than that determined at implant. Thus, if left unchecked, the safety margin determined necessary at implant is extremely wasteful during the chronic phase.
State of the art pacemakers now include an automatic capture system (see, for example the AUTOCAPTURE(trademark) pacing system used by Pacesetter, Inc., which, after implant of the pacemaker, automatically determines the capture-determining threshold and sets the stimulation pulse energy accordingly. The automatic capture system also periodically redetermines the capture-determining threshold and re-sets the stimulation pulse energy. Hence, if the threshold increases with time, the stimulation pulse energy is increased as needed to maintain capture. Alternatively, if the threshold decreases with time, the stimulation pulse energy is decreased as needed so that energy is not needlessly wasted.
A typical automatic capture system operates by applying a sequence of stimulation pulses to the heart tissue with differing pulse energy amounts and determines the lowest stimulation pulse energy sufficient to effectuate capture. To determine if capture has been effectuated by a stimulation pulse, the automatic capture system looks for an evoked response (ER) following the pulse. If no evoked response is detected, the pacemaker thereby concludes that the stimulation pulse did not have sufficient energy to effectuate capture. If an evoked response is detected, however, the pacemaker thereby concludes that the stimulation pulse had sufficient energy to effectuate capture.
More specifically, when capture occurs, the evoked response is represented by an intra-cardiac P-wave or R-wave (which typically has a different morphology, or wave shape, than does an intrinsic P-wave or R-wave which results from natural cardiac contractions) that indicates contraction of the respective cardiac tissue in response to the applied stimulation pulse. For example, using such an evoked response technique, if a stimulation pulse (hereafter referred to as a V-Pulse) is applied to the ventricle, any response sensed by the ventricular sensing circuits of the pacemaker immediately following the application of the V-Pulse is assumed to be an evoked response that evidences ventricular capture. Similarly, if a stimulation pulse (hereinafter referred to as an A-Pulse) is applied to the atrium, any response sensed by the atrial sensing circuits of the pacemaker immediately following the application of the A-Pulse is assumed to be an evoked response that evidences atrial capture. A specific automatic capture system is described in detail within U.S. Pat. No. 5,417,718 (Kleks et al.) which is incorporated by reference herein.
One problem with evoked response detection is that the signal sensed by the ventricular and/or atrial sensing circuits immediately following the application of a V-Pulse and/or A-Pulse may not be an evoked response. Rather, it may be noise, either electrical noise caused, for example, by electromagnetic interference (EMI), or myocardial noise caused by random myocardial or other muscle contractions (muscle xe2x80x9ctwitchingxe2x80x9d). Alternatively, that which is sensed by the ventricular and/or atrial sensing circuits may be a natural intrinsic R-wave or P-wave that just happens to occur immediately following the application of the non-capturing V-Pulse or A-Pulse.
Another signal that interferes with the detection of an evoked response, and potentially the most difficult to deal with because it is usually present in varying degrees, is lead polarization. Lead polarization is caused by electrochemical reactions that occur at the lead/tissue interface due to the application of the electrical stimulation pulse, A-Pulse or V-Pulse, across such interface. (The lead/tissue interface is that point where the electrode of the pacemaker lead contacts the cardiac tissue. Such a point is normally inside the atrium or the ventricle, assuming endocardial stimulation leads are employed.) Unfortunately, because the evoked response is sensed through the same electrode through which the A-Pulse or V-Pulse is delivered, the resulting polarization signal present at such electrode can corrupt the evoked response sensed by the sensing circuits of the pacemaker. To make matters worse, the lead polarization signal is not easily characterized. It is a complex function of the lead materials, lead geometry, tissue impedance, stimulation energy, and many other variables, most of which are continually changing over time.
In each case, the result is the samexe2x80x94a potentially false positive detection of the evoked response. Such a false positive detection leads to a false capture indication, which in turn can lead to missed heartbeats, a highly undesirable situation. Accordingly, techniques have been developed for measuring the amount of polarization and then adjusting the detected evoked response using the measured polarization to thereby reduce, or eliminate, the adverse effect of lead polarization on the evoked response.
Insofar as polarization measurements are concerned, state-of-the-art pacemakers typically measure the polarization in vivo by applying two pacemaker pulses to the heart separated by a time less than the natural refractory period of the heart. (The natural refractory period of the heart is that time period following depolarization or contraction of the cardiac tissue during which the cardiac tissue is not capable of depolarizing again. The natural refractory period, which may be thought of as a repolarization period, may vary from 100-200 milliseconds or more.) Typically, the two pulses are applied with a separation of about 60-90 milliseconds. The evoked response is measured after the first pulse in a window of approximately 50 milliseconds that occurs following a blanking window of 6-15 milliseconds. The second pulse occurs while the heart tissue is refractory. Polarization is also measured during the measurement window of about 50 milliseconds following the second pulse but still within the refractory period of the first pulse.
FIG. 1 illustrates a prior art method for measuring polarization based upon two pulses. The first pulse 100 triggers an evoked response 102. The evoked response is measured during a measurement window of 50 milliseconds following a blanking period of 10 milliseconds. Note that the measured evoked response 102 is actually a combination of the actual evoked response and some amount of polarization. To determine the true amount of evoked response by eliminating the polarization component, a second pulse 104 is applied following the evoked response measurement window. Any electrical response occurring following the second pulse is measured during a polarization measurement window of 50 milliseconds immediately following the second pulse. In the example of FIG. 1, a small amount of polarization 106 occurs. The amount of polarization measured is then used to adjust the evoked response amount detected during the evoked response measurement window. The final resulting evoked response amount is employed in programming the automatic capture system of the pacemaker.
It has been found that excessively high polarization readings may occasionally occur, even with leads that typically generate relatively little polarization. It is believed that the high readings are not representative of the true polarization but may be the result of evoked response bleed-through which occurs if the evoked response from the first pulse has not decayed sufficiently prior to measurement of polarization. Other factors that may affect polarization measurements performed following the second pulse include the amplitude of the evoked response, morphology, T-wave amplitude and sense amplifier characteristics.
FIG. 2 illustrates a situation, observed in the prior art, where evoked response bleed-through occurs. Within FIG. 2, a first pulse 110 is applied triggering an evoked response 112. The evoked response is measured during a measurement window of 50 milliseconds in length following a blanking period of 10 milliseconds. A calibration pulse 114 is applied at the end of the measurement window. However, the evoked response has not completely decayed prior to application of calibration pulse 114. Accordingly, any measurements made during a polarization measurement window immediately following the calibration pulse will detect both the polarization and the residual evoked response. As a result, the polarization amount is over-estimated and, when employed to adjust evoked response, the resulting value for the evoked response is incorrect. Hence, the automatic capture mechanism of the pacemaker could be set to an incorrect value.
Accordingly, within typical state-of-the-art pacemakers, the amount of polarization is compared against a maximum expected polarization threshold and, if it exceeds that threshold, the polarization is assumed to be inaccurate and the automatic capture function is disabled, thereby requiring manual threshold setting by a physician.
Another problem that may arise with some conventional polarization measurement techniques is that evoked response bleed-through may also arise based upon responses from intrinsic events. In this regard, it is possible that the second polarization measurement pulse may be delivered following an intrinsic event such as a P-wave or R-wave. If so, some portion of the response from the intrinsic event may affect the polarization measurement. Indeed, as a result of the additional response bleed-through from the intrinsic event, the measured polarization may exceed the cutoff threshold causing the automatic capture function to be disabled.
Hence, in at least some circumstances, the many advantages of the automatic capture system cannot be realized as a result of potentially inaccurate polarization measurements. Accordingly, it would be highly desirable to provide an improved technique for measuring polarization which permits a more accurate and reliable polarization measurement to be made, particularly in circumstances where an initial polarization measurement appears to be inaccurate, and it is to this end that aspects of the invention are primarily directed.
The invention provides a method and apparatus for determining the amount of polarization occurring following generation of electrical pulses in heart tissue connected to an implantable cardiac stimulation device. In accordance with a first aspect of the invention, a magnitude of electrical polarization is measured within the heart tissue using a first pair of electrical pulses separated by a first amount of time. Then, if the magnitude of electrical polarization exceeds a predetermined threshold, the magnitude is re-measured using a second pair of electrical pulses separated by a second amount of time, different from the first.
In this manner, if a first polarization measurement is high and may be erroneous, a second polarization measurement is automatically made with a greater amount of delay between the primary pulse and the subsequent calibration pulse. If the high initial polarization measurement was the result of evoked response bleed-through, the second measurement should be considerably lower and provide a better estimate of the true polarization. If the second polarization measurement is also high, then the automatic capture system or other features of the pacing device which rely on accurate polarization measurements are disabled. In an exemplary embodiment, the amount of time separating the first pair of pulses is between 60 and 90 milliseconds. The amount of time separating the second pair of pulses is between 40 and 200 milliseconds.
In accordance with a second aspect of the invention, rather than employing two pairs of electrical pulses, a sequence of three consecutive pulses are applied: a primary pulse, a first calibration pulse, and a second calibration pulse. The second calibration pulse is only administered if a polarization measurement made following the first calibration pulse exceeds the predetermined threshold. If so, the second calibration pulse is administered within the refractory period of the primary pulse and a second polarization measurement is made following the second calibration pulse. The second polarization measurement is employed in connection with setting the automatic capture system or other pacing functions requiring an accurate polarization measurement.
In accordance with a third aspect of the invention, the magnitude of electrical polarization is measured within the heart tissue using a polarization calibration pulse administered during a refractory period between an intrinsic depolarization event and a subsequent repolarization event. In one example of the method, an intrinsic QRS complex is detected and then a polarization calibration pulse is administered following a delay period to place the calibration pulse within the refractory period between the QRS complex and the subsequent T-wave. The magnitude of electrical polarization is then measured following the polarization calibration pulse.
Hence, rather than providing a pair of pulses with the first pulse triggering an evoked response and the second pulse triggering a polarization response, the evoked response is generated naturally within the heart. Thus, any possible adverse biological effects that might result from artificial stimulation of an evoked response are avoided.
In a further aspect of the present invention, the delays associated with the delivery of the calibration pulses may be automatically varied, e.g., increased, in order to search for the lowest polarization reading.
Other embodiments may be provided consistent with general principles of the invention. Apparatus embodiments are also provided.